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Abstract:

An apparatus for discrimination between a first (308) and a second
surface (304) type based on reflectivity has a light source (416) that
illuminates on a media surface. A photosensor (420) receives and measures
the reflection value from the surface. A first gain element adjusts a
voltage from the photosensor and a second gain element adjusts a current
measurement supplied to the light source. A subtractor (530) for
subtracting the first adjusted voltage (534) and the adjusted measurement
(538) are subtracted to provide an output value close to zero with
respect to the second surface and near a maximum with respect to the
first surface. The adjusted reflection value and a threshold reference
value (428) are compared (124) and indicates whether the first surface or
the second surface is present.

Claims:

1. An apparatus for discriminating between a first surface type and a
second surface wherein said first surface type reflectivity is higher
than said second surface type reflectivity comprising: a light source for
illuminate said first surface type or said second surface type; a
photosensor for measuring the reflectivity of said first or second
surface type; a first gain element for adjusting a voltage or current
from said photosensor based on said measured reflectivity; a second gain
element for adjusting a measurement of a current or a voltage supplied to
said light source; a subtractor for subtracting the first adjusted
voltage or current from the first gain element and the adjusted
measurement from the second gain element to provide a subtractor output
value; wherein said subtractor output value is close to zero with respect
to said second surface type and said subtractor output value is near a
maximum with respect to said first surface type; a threshold value
generator for generating a threshold reference value; and a comparator
which compares said adjusted reflection value and said threshold
reference value and indicates whether said first surface type or said
second surface type is present.

2. The apparatus according to claim 1 wherein said light source is a LED.

3. The device according to claim 1 wherein said first surface is an
emulsion covered surface.

4. The device according to claim 1 wherein said second surface is a slip
sheet paper.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Reference is made to commonly-assigned copending U.S. patent
application Ser. No. ______ (Attorney Docket No. 96545/NAB), filed
herewith, entitled METHOD FOR DISCRIMINATING BETWEEN OBJECTS, by
Burkatovsky; the disclosure of which is incorporated herein.

FIELD OF THE INVENTION

[0002] The present invention relates in general to an apparatus for
discriminating between two objects based on surface reflectivity
differences, and more specifically to discriminating between printing
plates covered by a polymer emulsion and interleaf paper between the
plates.

BACKGROUND OF THE INVENTION

[0003] A computer-to-plate (CTP) device 204, shown in FIG. 2, is used for
direct imaging on printing plates. The plates are loaded in a magazine or
cassette and delivered one by one to be exposed by the imaging device.
Alternatively the plates can be provided by an automatic plate loader
(APL) 104, wherein a plates stack 108 is inserted into the APL as is
shown in FIG. 1. The plates 304 provided in the cassette or in a plate
stack 104 are usually separated by interleaf paper 308, shown in FIG. 3,
interposed between the plates to prevent the emulsion-covered surfaces of
the plates from being damaged.

[0004] In the course of imaging plates, the plate placed at the top of the
stack is picked and transferred to the exposure area for imaging. When an
interleaf paper (slip-sheet) is at the top of the stack, the paper is
picked and disposed of, before picking the plate. There is a need for
discriminating between plate and the interleaf paper are used, to
correctly identify the topmost object on the stack.

[0005] U.S. Pat. No. 6,825,484 (Burkatovsky) describes a discriminating
device based on measurements of the light reflections from the surfaces
with different roughness. For example, discrimination between paper and
non-covered by emulsion printing plate will be reliable due to
substantially different roughness of paper as opposed to a smooth and
glossy plate metal surface. But discrimination between paper and emulsion
covered plate will often be inaccurate due to the small difference
between their roughness properties.

[0006] Another method for discriminating the slip sheets and emulsion
covered printing plate described in U.S. Pat. No. 7,157,725 (Kawamura).
This method is based on the difference between absorbance (reflectance)
of a slip sheet and a plate, irradiated by light of 570-740 nm
wavelengths.

[0007] Reflectance of emulsions and papers produced by different
manufacturers may vary substantially. An example of reflections from
papers and emulsions of different manufacturers is shown in FIG. 6.
X-axis depicts lighting source current or irradiating intensity T and
Y-axis depicts the reflection intensity represented by photosensor output
voltage Vr. The reflections from paper slip sheet and emulsion covered
plate manufactured by the first manufacturer and measured at
predetermined height and light source current is shown by lines 604 and
608 respectively. Reflections from paper slip sheet and emulsion
manufactured by the second manufacturer and measured at the same height
and light source current is shown by lines 612 and 616. The threshold
Vth1 for discrimination between paper and emulsion produced by the first
manufacturer according to Kawamura et al., should have value greater than
emulsion reflection and less than paper reflection values. While
irradiation caused by the same light source current the reflections from
paper and emulsion produced by the second manufacturer are smaller. This
might happen due to different processes applied for emulsion covered
printing plates and slip sheet by different manufacturers. In this case
the chosen threshold Vth1 will be greater than paper and emulsion
reflections and media discriminating will be impossible.

[0008] It should be noted that not only manufacturer media variations and
differences between the batches of media lead to reflection deviations.
Changing parameters such as distance to media, light source, ambient
light are also impact on reflections thus making difficult to practical
implementation of the method suggested by Kawamura et al.

[0009] The purpose of this invention is to improve the paper slip sheet
and emulsion covered plate discrimination capability.

SUMMARY OF THE INVENTION

[0010] Briefly, according to one aspect of the present invention an
apparatus for discrimination between a first surface type and a second
surface type is based on the first surface type reflectivity higher than
a second surface type reflectivity:

[0011] a) a light source adapted to illuminate light on a media surface,
the media surface is made from the first surface type or from the second
surface type;

[0012] b) a light source driver adapter to control the intensity applied
on the light source;

[0013] c) a photosensor adapted to receive and measure the reflection
value from the media surface;

[0014] d) a first gain element adapted to adjust the received reflection
intensity from the photosensor;

[0015] e) a second gain element to receive part of the light source
intensity and adjust it;

[0016] f) a subtractor adapted to receive a first adjusted value from
first gain element value and a second adjusted value from second gain
element value and subtract the second adjusted value from the first
adjusted value to create a subtractor output value wherein the subtractor
output value is close to zero in response to the second surface type and
the subtractor output value is maximal in response to the first surface
type;

[0018] h) a comparator adapted to compare between the adjusted reflection
value and the threshold reference value to create a discrimination
decision between the first surface type or the second surface type.

[0019] The invention and its objects and advantages will become more
apparent in the detailed description of the preferred embodiment
presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a schematic illustration of an automatic plate loader
(APL) loaded with stack of plates (prior art);

[0027]FIG. 8 is shows the discrimination behavior of the suggested device
in response to the distance from the measured target.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In the following detailed description, numerous specific details
are set forth in order to provide a thorough understanding of the
disclosure. However, it will be understood by those skilled in the art
that the teachings of the present disclosure may be practiced without
these specific details. In other instances, well-known methods,
procedures, components and circuits have not been described in detail so
as not to obscure the teachings of the present disclosure.

[0029] While the present invention is described in connection with one of
the embodiments, it will be understood that it is not intended to limit
the invention to this embodiment. On the contrary, it is intended to
cover all alternatives, modifications, and equivalents as covered by the
appended claims.

[0030] The schematic illustration of a discriminating device known the art
is shown in FIG. 4. FIG. 5 shows components of the proposed invention,
where some of the components are also being used in the Kawamura et al.,
as are shown in FIG. 4. The illumination control unit 404 intended for
setting the level of media 408 illumination connected through the light
source driver 412 to the light source 416, the light source can be a LED
(light emitting diode). The reflections from media 408 are measured by
photosensor 420.

[0031] An adjustment element 542 is connected by its inputs to the outputs
of the photosensor 420 and the illumination control unit 404. The output
of the adjustment element 542 is connected to the first input of
comparator 124 while the second input of comparator 124 is connected to
the threshold reference 428.

[0032] The irradiation of the tested media provided by the light source
416 controlled by the illumination set point signal Vi produced by
illumination control unit 404 through the light source driver 412. The
photosensor 420 generates signal VR proportional to the reflection from
tested media. This signal is amplified by first input amplification
factor 534 (GR) of adjustment element 542, simultaneously the
illumination set point signal Vi is gained (divided) by second input
amplification factor 538 (Gi) and than subtracted by subtractor 530 from
the gained VR signal. The result of the subtraction is represented by
subtractor output value signal Vs complying with following equation:

Vs=VR*GR-Vi*Gi (1)

FIG. 7 explains the selection of GR and Gi which is substantial for
suggested discriminating device. Assuming GR0=1 and Gi0=0.

[0033] In this case according to Equation (1) VS will equal to VR.

Vs=VR (2)

In the case when VS equals VR, according to Equation (1), the behavior of
the proposed discrimination device shown in FIG. 5 will behave as the
device described by Kawamura et al, which is shown in FIG. 4.

[0034] Line 704 in FIG. 7 shows a response function representing the
reflection from paper slip sheet and line 708 represents reflection from
emulsion covered plate surface respectively. These lines are identical to
lines 604 and 608 of FIG. 6. Respectively Vsp=Vpl and Vse=Vel. The
relation between the Vsp and Vse determines the capability of the device
to perform a reliable discrimination between paper slip sheets and
emulsion covered plates. The relation between amplified paper reflection
and amplified (divided) emulsion reflection is defined as discrimination
factor DF.

DF=Vsp/Vse (3)

[0035] As much as the Vsp value is bigger than Vse value, the
discrimination will be more reliable, due to covering of a larger
reflection range and thus decreasing the sensitivity of reflection
deviations.

[0036] In other words in order to improve the discrimination capability of
a discrimination device the value of DF needs to be increased. This can
be achieved by adjusting the amplification factors 534 (GR) and 538 (Gi).
The Gi adjustment should be provided while emulsion covered printing
plate is examined. Adjustment may start with mentioned above values of
Gi0 and GR0. (Gi0=0, GR0=1). According to equations (2) and (3)
discrimination factor for these values will be defined as

DF0=Vsp0/Vse0. (4)

[0037] Now by increasing 538 (Gi) up to the moment when Vse will be close
to zero we obtain the situation when Vse is practically not dependent
upon the light source 416 current and remains low within the light source
current possible range (line 716). Respectively after adjusting 538 (Gi)
while examining the paper slip sheet, the Vsp line 704 will change its
slope. The Vsp dependence on light source 416 current after Gi adjustment
is presented by line 712.

[0038] Increasing the light source 416 current to Im by means of
illumination control unit 404 we obtain Vsph value while examining paper
and Vsel value while examining emulsion covered plate. As Vsph is bigger
than Vsp and Vsel is lower than Vse thus according to equation (3) the
value of representing discrimination factor

DF1=Vsph/Vsel (5)

will be much bigger than DF0 (4), thus yielding a substantially improved
discrimination capabilities. The maximum value of Vsph is restricted by
power supply voltage. In other words the threshold margin is enough to
support the discrimination of plates and emulsions from various
manufacturers.

[0039] It should be noted that the DF1 value may be achieved also without
light source 416 current changing (from In to Im). This can be obtained
by increasing the amplification factor 534 (GR) while maintaining line
716 close to the X-Axis (as is shown in FIG. 7), by adjusting the
amplification factor 538 (Gi).

[0040]FIG. 8 depicts the subtractor output value Vs signal used in FIG. 7
as a function of media to sensor distance shown as axis H (804).
Comparing to FIG. 7 point Hn of axis H determines the media to sensor
distance while illuminating current value is In. Reflections from paper
slip sheet and emulsion covered plate at this point are the same as shown
on FIG. 6 (Vpl and Vel). Respectively the subtractor output values while
Gio=0 and GRo=1 are Vsp and Vse, the same as shown on FIG. 7. Now while
maintaining the constant value of LED current In the amplification
factors 534 (GRo) and 538 (Gio) should be adjusted such as subtractor
output value Vs is close to zero Vsel in response to lower reflectance
surface type (emulsion covered plate). As a result of the subtractor
output value Vs is maximal Vsph in response to higher reflectance surface
type (slip slit paper). The subtractor output value behavior represents
the emulsion covered plate reflection after gains adjustment is depicted
by line 812. Respectively the subtractor output value behavior for the
slip sheet paper reflection after gains adjustment is represented by
behavior function 808 (sensor to media distance is bigger than Hn) and
line Vsph--maximum voltage value restricted by power supply voltage
(sensor to media distance is smaller than Hn).

[0041] As it can be seen from FIG. 8 the range from smaller media to
sensor distance Hm to higher media to sensor distance Hn shows
practically same subtractor output values such as Vsel close to zero in
the case of emulsion testing and Vsph close to power supply voltage in
case of slip sheet paper testing. Within this (Hm-Hn) range the
discrimination factor has maximum allowable value DF=(Vsph/Vsel)
according to Equation (3) and DF=constant as well. Referring to the prior
art performance according to FIG. 8, in the Hn sensor to media distance
point the prior art device will have a worse DF than the suggested device
as is shown by Equations (4) and (5). In addition the Hm sensor to media
distance point in prior art device will not work at all, due to very high
incoming reflection. Specific gains adjustment allows discrimination
performance practically independent of sensor to media distance and
extending of sensor to media distance range where discriminating is
possible.

[0042] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within the
scope of the invention.